1. Field of the Invention
This invention relates to apparatus for producing electromagnetic radiation of three different wavelengths, especially red, green and blue light, using an optical parametric oscillator. It also relates to a method for producing light of at least three different wavelengths using an optical parametric oscillator.
Light sources of red, green and blue light (RGB systems) may comprise a laser primary light source and an optical parametric oscillator (OPO) for nonlinear conversion. More specifically, the OPO is usually synchronously pumped with picosecond or femtosecond pulses. This means that the OPO cavity usually must have the same length as the cavity of the primary laser or an integer fraction or multiple of this length. The OPO cavity length must be precisely stabilized to match the laser cavity length within typically a few tens of micrometers, depending on the pulse durations. Particularly with femtosecond pump pulses, the stabilization becomes rather critical and is seen as an important drawback of synchronously pumped OPOs.
An OPO consists of a similar cavity as a laser resonator cavity, but the gain in the OPO is generated in a nonlinear crystal (e.g., made of LBO, KTA etc.) which is pumped with the pulses from a laser. The nonlinear crystal of the OPO generates a signal wave, for which the OPO cavity is resonant, and a corresponding idler wave. Alternatively, the OPO cavity may be resonant for the idler wave. The circulating signal pulses are synchronized with the pump pulses. The wavelengths of signal and idler waves are determined by phase matching, which depends on the refractive indices of the nonlinear crystal, i.e., on the material, its temperature and the propagation directions. It has been shown (L. Lefort et al., Opt. Lett. 24 (1), 28, 1999) that the pulses generated by a synchronously pumped OPO can be more than 10 times shorter than the pump pulses.
2. Description of the Related Art
High average power picosecond or femtosecond laser systems are known in the art. WO 01/43242 discloses a so-called “thin-disk” laser being an ultrafast high average power system. A high average power picosecond laser system is described in Applied Phys. B, Vol. 17, pp. 19–25 (2000).
U.S. Pat. No. 5,828,424 discloses a combination of conversion stages for generating three different visible light wavelengths using a mode coupled Nd:YLF laser primary source with a wavelength of 1053 nm or 1047 nm. The apparatus of this patent, however, suffers from severe drawbacks. For example, the schemes anticipated in the patent all use nonlinear crystals (e.g., KTA) and/or phase matching schemes that require operation at elevated temperatures. Thus, elevated temperature control means are required. Also, the disclosed nonlinear crystals are somewhat expensive and difficult to obtain and tend to be damaged if the apparatus is run for a long time with high power.